A mobile communication system generally refers to any telecommunication system which enables wireless communication when a user is located within the service area of the system. Examples of such systems are cellular mobile communication systems, such as the GSM (Global System for Mobile communications), or corresponding systems, such as the PCS (Personal Communication System) or the DCS 1800 (Digital Cellular System for 1800 MHz), third-generation mobile systems, such as the UMTS (Universal Mobile Communication System), and systems based on the above mentioned systems, such as GSM 2+ systems and future fourth-generation systems. One typical example of a mobile communication system is a public land mobile network PLMN.
In addition to normal speech and data connections set up on traffic channels in digital mobile communication systems, short digital data messages comprising user data can be transmitted between actual signalling on control and signalling channels of a system. The main reason for this is to avoid frame stealing from the traffic channel. These short digital data messages comprising user data are generally referred to as short messages. In the GSM, control channels used for short message transmissions are the SACCH used for transmission of measurements performed by a mobile station MS during the connection and an SDCCH (Stand alone Dedicated Control Channel) carrying out the actual signalling. If a mobile station has a connection on a traffic channel, short messages are forwarded on the SACCH, otherwise they are forwarded on the SDCCH.
In the GSM, short message service has been implemented by using LAPDm (Link Access Protocol for the Dm channel) protocol in the data link layer, i.e. layer 2. The LAPDm provides services to an upper layer, layer 3, via Service Access Points (SAP), which are identified by a Service Access Point Identifier (SAPI). The LAPDm has two different values for the SAPI. Signalling is transferred in SAPI=0 frames using unacknowledged mode operation on the SACCH associated with a traffic channel. Short messages are transferred in SAPI=3 frames using acknowledged mode operation on the SACCH associated with a traffic channel. The LAPDm acknowledged mode operation prioritizes messages from a layer 3 entity, so that if a SAPI=3 frame, i.e. a short message, is waiting for transmission, two SAPI=0 frames are not sent in consecutive frames. It is also ensured that at least one SAPI=0 frame is transmitted after a SAPI=3 frame. In other words, short messages are transmitted on the SACCH associated with a traffic channel by pre-emption of every second signalling frame, i.e. SAPI=0 frame.
However, in a GSM 2+-system called GERAN (GSM/EDGE (Enhanced Data rates for Global Evolution) Radio Access Network) Release 5, it has been suggested to use the RLC/MAC protocol instead of the LAPDm in order to make the protocol stacks less complicated. One of the problems associated with the replacement of the LAPDm with the RLC/MAC is that there is no support in the RLC/MAC for the pre-emption mechanism illustrated above.